In electrostatographic imaging and recording processes such as electrophotographic copying, an electrostatic latent image formed on a photoconductive surface is developed with a thermoplastic toner powder, which is thereafter fused to a receiver. The fusing member can be a roll, belt, or any surface having a suitable shape for fixing thermoplastic toner powder to the receiver. The fusing step commonly consists of passing the receiver, for example, a sheet of paper on which toner powder is distributed in an imagewise pattern, through the nip of a pair of rolls. At least one of the rolls is heated; in the case where the fuser member is a heated roll, a smooth resilient surface is bonded either directly or indirectly to the core of the roll. Where the fuser member is in the form of a belt, it is preferably a flexible endless belt having a smooth, hardened outer surface that passes around the heated roller.
A toner fuser roll includes a cylindrical core, often metallic, that typically has a heating source in its interior. A resilient base cushion layer, which may contain filler particles to improve mechanical strength and/or thermal conductivity, is formed on the surface of the core, which may advantageously be coated with a primer to improve adhesion of the resilient layer. Roll cushion layers are commonly made of silicone rubbers or silicone polymers such as, for example, poly(dimethylsiloxane) PDMS) polymers of low surface energy, which minimize adherence of toner to the roll.
Frequently, release oils composed of, for example, poly(dimethylsiloxanes) are also applied to the fuser roll surface to prevent the toner from adhering to the roll. Such release oils may interact with the PDMS in the resilient layer upon repeated use, which in time causes swelling, softening, and degradation of the roll. To prevent these deleterious effects caused by release oil, a thin banier layer of, for example, a cured polyfluorocarbon, is formed on the cushion layer.
Electrophotography can be used to create photographic quality multicolor toner images when the toner particles are small, that is, less than about 10 m, and the receivers, typically papers, are smooth. A typical method of making a multicolor toner image involves trichromatic color synthesis by subtractive color formation. In such synthesis, successive imagewise electrostatic images, each representing a different color, are formed on a photoconductive element, and each image is developed with a toner of a different color. Typically, the colors correspond to each of the three subtractive primary colors (cyan, magenta and yellow) and, optionally, black. The imagewise electrostatic images for each of the colors can be made successively on the photoconductive element by using filters to produce color separations corresponding to the colors in the image. Following development of the color separations, each developed separation image can be transferred from the photoconductive element successively in registration with the other color toner images to an intermediate transfer member. All the color toner images can then be transferred in one step from the intermediate transfer member to a receiver, where they are fixed or fused to produce a multicolor permanent image. To match the photographic quality produced using silver halide technology, it is desirable that these multicolor toner images have high gloss.
In the fusing of the toner image to the receiver, the area of contact of the fuser roll with the toner-bearing surface of the receiver sheet as it passes through the nip of the pair of rolls is determined by the amount pressure exerted by the pressure roll and by the characteristics of the resilient cushion layer. The extent of the contact area helps establish the length of time that any given portion of the toner image will be in contact with and heated by the fuser roll.
As previously mentioned, PDMS cushion layers may include fillers comprising inorganic particulate materials, for example, metals, metal oxides, metal hydroxides, metal salts, and mixtures thereof. For example, Fitzgerald U.S. Pat. No. 5,292,606, the disclosure of which is incorporated herein by reference, describes fuser roll base cushion layers that contain fillers comprising particulate zinc oxide and zinc oxide-aluminum oxide mixtures. Similarly, Fitzgerald U.S. Pat. No. 5,336,539, the disclosure of which is incorporated herein by reference, describes a fuser roll cushion layer containing dispersed nickel oxide particles. Also, the fuser roll described in Fitzgerald et al. U.S. Pat. No. 5,480,724, the disclosure of which is incorporated herein by reference, includes a base cushion layer containing 20 to 40 volume percent of dispersed tin oxide particles.
Filler particles may also be included in a barrier layer. For example, Chen et al. U.S. Pat. No. 5,464,698, the disclosure of which is incorporated herein by reference, describes a toner fuser member having a silicone rubber cushion layer and an overlying layer of a cured fluorocarbon polymer in which is dispersed a filler comprising a particulate mixture that includes tin oxide.
There remains a need for toner fuser rolls that resist degradation by release agents and have good thermal conductivity, and also produce fused images, including multicolor images, of high gloss. The toner fuser roll of the present invention, which is characterized by a highly smooth outermost layer with excellent wear properties, meets this need.